signal

Signal

a sign, physical process, or phenomenon that carries a message about some event or the state of an object or that conveys, for example, a control command or a warning. Any event, however complex, can be represented to a given degree of completeness by a set of signals. According to their nature, signals can be classified in such groups as the following: mechanical signals, such as a deformation or a change in pressure; thermal signals, such as a change in temperature; light signals, which may be flashes of light or visual images; electric signals, which consist of changes in current or voltage; electromagnetic signals, such as radio waves; and sound signals, which consist of sound vibrations.

The information contained in the message is usually represented by a change in one or more parameters of the signal—for example, in the signal’s amplitude (intensity), duration, frequency, width of spectrum, phase, lag time, or polarization (seeMODULATION OF LIGHTandMODULATION OF OSCILLATIONS). Signals can be converted, without changing the information they carry, from one type to another type that is more convenient for subsequent transmission, perception, storage, or processing or that is more suitable for a subsequent purposeful change in the information contained in the message. The conversion of continuous signals into discrete signals is known as signal quantization; in this case, some loss of information is inevitable.

An example of signal conversion is presented by a magnetic sound recording of a musical performance on a piano. The note signs are visually perceived by the pianist as light signals, and he reproduces them by pressing the keys of the piano (mechanical signals). The resulting vibrations of the strings are accompanied by sound vibrations of different frequencies (sound signals). A microphone converts the sound waves into changes in current in a circuit (electric signals). This current induces in the core of a magnetic head an alternating magnetic field (electromagnetic signals), which causes the remagnetization of sections of a magnetic tape and thereby produces the recording proper.

The use of a given signal depends on a number of factors, including the requirements of the specific message-transmission task with regard to, for example, amount of information, speed of transmission or processing, transmission reliability, transmission quality, transmission accuracy, and the noise immunity of the communication channel. Other determining factors are the level and nature of the noise and the feasibility of the receiving and transmitting systems. In radio communication and broadcasting systems, for example, amplitude- or frequency-modulated sinusoidal electric oscillations are used as signals. Transportation signaling systems make use primarily of light signals, such as changes in color or flashing lights and sound signals such as those produced by a horn or siren. For the most part electric, electromagnetic, and, to a lesser extent, light signals are used in the transmission of information over great distances, in the computer processing of information and in the radar and navigational systems of ships and aircraft. Such signals are characterized by their base, which is the product of the width of the signal’s spectrum and the duration of the signal. If the base of the signal is ~ 1, then the signal is said to be simple; if the base is ≫1, then the signal is compound. An important signal parameter in some fields of application, such as radar, is the correlation (or autocorrelation) function (seeCORRELATIONandCORRELATION ANALYSIS). This function characterizes the rate of change of the signal at the output of an optimum detector—that is, a detector matched with the signal—when a change occurs in the frequency or lag time of the input signal. The correlation function is used chiefly to assess the accuracy and resolution of the radar station with respect to target speed and range. For pulsed signals (seePULSE ENGINEERING), the on-off ratio is an important parameter. In sound recording equipment and measuring equipment, signals of nonelectric origin are generally converted into electric signals, as the signals most convenient for such operations as transformation, amplification, and correction.

The concept of a signal was first defined precisely in cybernetics, which drew attention to the four components invariably present in a signal that carries information on a specific event. These components are the physical carrier of the signal, the form of expression of the signal (syntactics), the meaning of the signal (semantics), and the rules governing the attribution of various meanings to the same signal (pragmatics). Semiotics deals with the establishment of the general rules and interrelations of syntactics, semantics, and pragmatics. Information theory studies the general rules governing the conversion and transmission of signals, without regard to the physical nature of the signals.

signal

(operating system)

A predefined message sent between two
Unix processes or from the kernel to a process. Signals
communicate the occurrence of unexpected external events such
as the forced termination of a process by the user. Each
signal has a unique number associated with it and each process
has a signal handler set for each signal. Signals can be sent
using the killsystem call.

signal

Any electrical or light pulse or frequency whether in a wire or fiber or wireless. The term is somewhat generic and may refer to virtually anything that is generated and transmitted (power, data, control signals). Although the term may be used by itself later on in a description or article, it is often used at least initially with another word, such as "carrier signal," "data signal" or "control signal." See signal-to-noise ratio and control signal.

Signal Vs. Signaling A "signal" can refer to a data signal or a control signal, but "signaling" refers to transmitting only control signals. It is very common to hear the term in the telecom industry, where "signaling" means setting up and breaking down a call, but does not refer to the actual pulses or frequencies of the data (voice, text, etc.) itself (see SS7 and signaling).

Methemoglobin, with its shortT1 relaxation time on T1-weighted imaging, is easily identified in the membranous labyrinth because of the distinct contrast between it and the surrounding bony signal void of the otic capsule.

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